Yesterday we experienced (Pun intended) a very strange behaviour when moving a Sitecore analytics database to a newly installed server.
Using the normal mongodump/mongorestore commands
mongodump --db Sitecore_Mongo_Analytics --out d:\mongodumps\ mongorestore --db Sitecore_Mongo_Analytics d:\mongodumps\
the data was backed up but when using mongorestore the database to the new server the data did not get restored. We only got an empty database.
It seems that MongoDB 3.2 is not compatible with MongoDB 3.0.9 (The source of the original backup)
Thanks to my collegue Lasse Aagaard Pedersen for helping solve that little mystery.